The solar system consists of the sun, the nearest star to us, and the planets Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto, as well as numerous comets, asteroids, meteoroids, and the interplanetary medium. The nine planets, along with their satellites, revolve around the sun in definite orbits. The planets and the satellites are said to be the members of the solar family or the solar system. It is believed that the sun, the center-piece of the solar system, was born five billion years ago, emitting heat and light non-stop, and it is expected to do so for the next five billion years. The sun is the richest repository of electromagnetic energy, in the form of heat and light. The sun contains 99.85 per cent of all the matter in the solar system. The planets, which were condensed out of the same disk of material that formed the sun, contain only 0.135 per cent of the mass of the solar system. The four innermost planets Mercury, Venus, Earth, and Mars are called terrestrial planets. They all have compact, rocky surfaces like the Earths. Jupiter, Saturn, Uranus, and Neptune are known as the Jovian (Jupiter-like) planets on account of their gigantic sizes. By volume, nearly all the solar system appears to be an empty void. However, this vacuum of space encompasses the interplanetary medium containing various forms of energy, dust, and gas. The flow of gas and charged particles called plasma, mostly protons and electrons, is known as the solar wind. Its speed is about 250 miles (400 kilometers) per second in the vicinity of the Earths orbit. The whole solar system, together with the stars visible on a clear night, orbits the center of our home galaxy called the Milky Way, which is a spiral disk of 200 billion stars. |
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